Preliminary studies suggest that thin filaments in young cultured mouse adrenal cells are involved in transporting cytoplasmic cholesterol to the mitochondria to initiate ACTH-stimulated steroidogenesis. After extraction of the plasma membrane of young adrenal cells with detergent, several different kinds of thin filament associations can be observed by electron microscopy. However, the extraction technique may preserve only certain of these associations; in addition, we do not know which of the associations may transport cholesterol. Finally, cholesterol-containing lipid droplets, and mitochondria, are lost during extraction; thus, determining thin filament-directed changes in cellular location of these organelles after ACTH stimulation is impossible. We propose using the high voltage transmission electron microscope to examine whole adrenal cells and extracted cytoskeleton filaments to obtain information regarding changes in thin filaments and steroidogenic organelles. We hope to examine cultured adrenal cells from other species to determine whether there are similar changes we can observe in these cells. Because older adrenal cells do not produce steroids we will examine old adrenal cells from each species for qualitative defects in the relationships of the thin filaments and steroidogenic organelles.